The present invention relates generally to a surface-treated paper product which provides a substrate for receiving a thermosensitive recording layer and which is suitable for use in low heat thermal printing operations. More particularly, the present invention is directed to such a substrate wherein a surface of a paper sheet or support is coated with a single layer of a pigment-containing, thermal insulating composition that is positionable between the paper sheet and the thermosensitive recording layer.
The utilization of heat sensitive recording materials in printing operations for providing selected images on a paper-supported heat sensitive layer through the use of heat pens or thermal printing heads such as used in facsimile machines is becoming of increasing importance in the communications field. Basically, image reproduction in facsimile machines and other thermal printers is achieved by passing a sheet of paper coated with a thermosensitive recording layer containing one or more layers of heat-fusible inks or a dye and a heat sensitive developer in a suitable binder in close proximity to a thermal printing head so that recording signals provided to the thermal printing head can direct selected patterns of heat onto the thermosensitive recording layer for creating the desired image therein.
During early stages in the development of heat sensitive recording papers, it was found that the thermal recording sensitivity of various thermosensitive recording layers on the paper could be enhanced by placing layers of fillers and pigments between the thermosensitive recording layer and the paper support. These intermediate layers improved image contrast, surface smoothness, and reduced heat transfer between the thermosensitive recording layer and the paper support. With the use of such intermediate layers the thermal printing head could be maintained in a close proximity to the thermosensitive recording layer during the printing operation while permitting a large part of the heat from the thermal printing head to be concentrated in the thermosensitive recording layer. These improvements in thermal sensitive recording papers resulted in increased recording speeds and image sharpness so as to render heat sensitive printing a more viable recording practice.
Heat sensitive recording papers employing one or more layers incorporating pigments, fillers and/or thermal insulating materials and which are located between a paper support and a thermosensitive recording layer are described in the literature including U.S. Pat. No. 4,798,820, issued Jan. 17, 1989, European Patent Application No. 0281315, published May 23, 1989, Japanese "Kokai" Patents Nos. Sho 60--284390, published Dec. 9, 1985, and Sho 62-5586, published Jan. 12, 1987, and Japanese Patent Publication No. 84-04520/80, published Jan. 11, 1984.
While these previous developments resulted in improved heat sensitive recording papers, these papers were found to be primarily useful in thermal printers utilizing relatively high levels of heat at the thermal printing heads for developing the thermal image. In as much as the majority of the thermal printers presently in use are of the types using medium and high heat at the thermal printing heads, these heat sensitive recording papers are currently in demand. However, when using these previously provided heat sensitive recording papers in thermal printers using relatively low heat levels at the thermal printing heads, the image density produced on the thermosensitive recording layer was often less than that obtained when using the same heat sensitive recording papers in medium and high heat thermal printers. Thus, with a recent trend in thermal printers moving towards the use of thermal printers using less heat at the thermal printing heads in order to promote faster printing speeds there is presently a need for a coated base paper, which when provided with a coating of a suitable thermosensitive recording composition, can provide a satisfactorily high level of image density when used in lower heat thermal printers.